Glass fibers are useful in a variety of technologies. For example, glass fibers are used as reinforcements in polymer matrices to form glass fiber reinforced plastics or composites. Glass fibers have been used in the form of continuous or chopped filaments, strands, rovings, woven fabrics, non-woven fabrics, meshes, and scrims to reinforce polymers. Glass fibers are commonly used as reinforcements in polymer matrices to form glass fiber reinforced plastics or composites because they provide dimensional stability as they do not shrink or stretch in response to changing atmospheric conditions. In addition, glass fibers have high tensile strength, heat resistance, moisture resistance, and high thermal conductivity.
Typically, glass fibers are formed by attenuating streams of a molten glass material from a bushing or orifice. An aqueous sizing composition containing a film forming polymer, a coupling agent, and a lubricant is typically applied to the fibers after they are drawn from the bushing to protect the fibers from breakage during subsequent processing and to improve the compatibility of the fibers with the matrix resins that are to be reinforced. After the sizing composition has been applied, the sized fibers may be gathered into separate strands and wound to produce a glass fiber package. The glass fiber package may then be heated to remove water and deposit the size as a residue lightly coating the surface of the glass fiber. Multiple numbers of the resulting dried glass fiber packages may be consolidated and wound onto a spool referred to as a roving doff or package. The roving package is composed of a glass strand with multiple bundles of glass fibers.
Reinforcement rovings may be used in a sheet molding compound (SMC) process. In an SMC production process, a layer of a polymer film, such as an unsaturated polyester resin or vinyl ester resin premix, is metered onto a plastic carrier sheet that has a non-adhering surface. Chopped glass fiber roving bundles are then deposited onto the polymer film and a second, non-adhering carrier sheet containing a second layer of polymer film is positioned onto the first sheet such that the second polymer film contacts the chopped glass bundles and forms a sandwiched material. This sandwiched material is then kneaded to distribute the polymer resin matrix and glass fiber bundles throughout the resultant SMC material, which may then be rolled for later use in a molding process.
In the production of SMC compounds, it is desirable that the chopped glass fiber bundles contact the polymeric matrix material. One measure of this contact is referred to as wetting, which is a measure of how well the glass bundles are encapsulated by the matrix SMC resin material. It is desirable to have the glass bundles completely wet with no dry glass. Incomplete wetting during this initial processing can adversely affect subsequent processing as well as the surface characteristics of the final composite. For example, poor wetting may result in poor molding characteristics of the SMC, resulting in low composite strengths and surface defects in the final molded part. The SMC manufacturing process throughput, such as lines-speeds and productivity, are limited by how well and how quickly the roving chopped bundles can be completely wet.
Thus, there exists a need in the art for a sizing composition that improves wetting, and correspondingly the SMC production rate and the physical properties of the molded composite part.